Real Dictionary

cactus
countable noun: A cactus is a thick fleshy plant that grows in many hot, dry parts of the world. Cacti have no leaves and many of them are covered in prickles.

cactus in American English
any desert plants of the cactus family, native to the New World, with fleshy stems, reduced or spinelike leaves, and often showy flowers
designating a family (Cactaceae) of dicotyledonous plants (order Caryophyllales), including the prickly pear, saguaro, and cereus
noun: any of numerous succulent plants of the family Cactaceae, of warm, arid regions of the New World, having fleshy, leafless, usually spiny stems, and typically having solitary, showy flowers

cactus in British English
noun: any spiny succulent plant of the family Cactaceae of the arid regions of America. Cactuses have swollen tough stems, leaves reduced to spines or scales, and often large brightly coloured flowers